rswitch: Add PM ops

Add PM ops for Suspend to Idle. When the system suspended,
the Ethernet Serdes's clock will be stopped. So, this driver needs
to re-initialize the Ethernet Serdes by phy_init() in
renesas_eth_sw_resume(). Otherwise, timeout happened in phy_power_on().

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>